It is known, from DE 12 76 912 C2 and CH 46 37 74 C1, that in steam pressing the mat is first compressed, then steam is introduced, and then it is further compressed. Thus, in EP 0 172 930 A1 a process for steam pressing has become known wherein the mat is first compressed without steam to 50% of the final thickness and then further steamed in various flushing steps. It is common to this patent family that the mat is first slightly compressed and then the entire cross section of the mat is heated by the introduction of steam to the steam temperature.
The boards made according to CH 46 37 74 C1 and the other older patents often had blow-outs, and dark spots were to be seen on the board surface. The board strength, especially the cross-tensional strength in the areas of the dark spots is considerably lower than in the rest of the board area. The dark spots are situated opposite holes out of which the steam flows. The fiber-to-glue structure is destroyed also by the high velocity of flow of the steam and the large amount of steam that flows through this area. A study has shown that the strength of the board is reduced particularly in the outer areas, since in this area the phenolic resin becomes washed out.
In U.S. Pat. No. 4,517,147 it is described how boards can be manufactured by steam pressing without blow-outs and traces of the steam holes in the board surface. U.S. Pat. No. 4,517,147 proposes a reduction of the steam velocity under the holes. This is to be achieved by the use of a special screen between the mat and the heating plate. Also, the mat density and the steam pressure are to be set low. In spite of the many proposals for a solution it has been impossible to date to press phenolic resin-treated wood chips to form OSB boards, because in OSB manufacture in multi-stage presses the pressing time is relatively long in comparison to other wood materials. Thus, the press factor for an OSB board 11 mm thick is 12 s/mm, while the press factor for a chip board amounts, for example, to only 6 s/mm.